The Behavior Science Core has a twenty year history echoing that of our MRDDRC as a whole. We have evolved from a relatively focused effort supporting half a dozen projects to a multifaceted resource that, this current funding period, has contributed to the productivity of over 85 projects. This history, together with our current plans, is briefly summarized here. Since its establishment in 1988, the Behavior Science Core has been engaged in hundreds of collaborations and experienced dynamic growth in its capabilities and resources, as would be expected given its mission of providing core support to a large and diverse program of research in our rapidly evolving field of mental retardation (MR) and developmental disabilities (DD). Initially, the Core focused on combining the expertise of applied behavior analysts with that of biomedical scientists interested in studying brain-behavior relationships, patterns of behavior and behavioral symptoms. Under the direction of Michael Cataldo, the Core provided expertise for studying how environmental variables relate to the prevention and reduction of behavior problems in individuals with MR/DD. Behavior analysis expertise was provided to facilitate the interaction between behavioral and biological investigators at the Kennedy Institute and the Johns Hopkins University School of Medicine. Drs. Cataldo and Keith Slifer, assisted by one Research Associate, originally staffed the Core. The services provided at that time focused on the design of behavioral observation systems, behavioral data collection and analysis. Behavioral measurement was accomplished using standard measures already established in the behavioral literature or by designing customized data collection approaches to meet the unique needs of MRDDRC researchers (e.g. Naidu, Hyman, Piazza et al., 1990;Piazza, Fisher, &Moser, 1991). During the initial funding period, the Core also provided behavior management services that employed principles of learning and behavior to maximize research participant cooperation with study protocols requiring neuroimaging (Reiss, Aylward, Freund et al., 1991;)and medication adherence (Babbitt, Parish, Brierly, Kohr, 1991). During the initial period of funding, six research projects and five investigators utilized the Core.